The present invention relates to a control system for a continuously variable belt-drive automatic transmission for a motor vehicle, and more particularly to a fail-safe system for a failure of sensors such as speed sensors of a pulley of the transmission.
A known continuously variable belt-drive transmission (hereinafter called CVT) for a motor vehicle comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulleys in dependency on driving conditions. The system is provided with a hydraulic circuit including a pump for supplying oil to servo devices, a line pressure control valve and a transmission ratio control valve. Each valve comprises a spool to control the oil supplied to the servo devices. The transmission ratio control valve operates to determine the transmission ratio in accordance with the opening degree of a throttle valve of an engine and the speed of the engine.
In a control system described in EP-A No 0182616, a drive pulley speed (engine speed) sensor, a driven pulley speed sensor, and a throttle position sensor are provided. Actual transmission ratio (i) is calculated by a calculator based on the output signals (N.sub.P) of the drive pulley speed sensor and the output signal (N.sub.S) of the driven pulley speed sensor with a formula (i=N.sub.P /N.sub.S). Desired transmission ratio (id) is calculated based on the output signal (N.sub.S) of the driven pulley speed sensor and the output signal (.theta.) of the throttle position sensor. The actual transmission ratio i is controlled to the desired transmission ratio id by shifting the spool of the transmission ratio control valve.
When failure occurs in the sensing system, such as failure of one of the sensors, or disconnection of a connector of a sensor, the transmission ratio can not be controlled. For example, if the connector of the drive pulley speed sensor disengages from a socket, the output signal N.sub.P of the sensor becomes zero. Accordingly, the output (i=N.sub.P /N.sub.S) of the calculator becomes a minimum value i min. In most cases, the desired transmission ratio id is larger than the minimum value i min. Therefore, the transmission is downshifted so that the actual ratio becomes the desired ratio id. As a result, the transmission ratio i becomes a maximum value i max. If the transmission is rapidly downshifted during the driving of the vehicle, the engine speed quickly increases to an excessive speed, which causes a breakdown of the engine in case of a high vehicle speed.
When the output signal N.sub.S of the driven pulley speed sensor becomes zero, the output (i=N.sub.P /N.sub.S) of the calculator becomes a maximum value i max. Accordingly, the transmission is upshifted so as to provided the desired value. As a result, the transmission ratio reaches a minimum value i min. The minimum value is kept during the driving. Namely, the vehicle is driven by very small torque, which will cause a reduction in the driveability of the vehicle.